Effect of temperature during burial on dormant and non-dormant seeds of Lamium amplexicaule L. and ecological implications

Spring-produced seeds of Lamium amplexicaule L. were dormant at maturity in May and after-ripened when buried and stored over a range of temperatures, becoming conditionally dormant at low (5, 15/6 and 20/10°C) and non-dormant at high (25/15, 30/15 and 35/20°C) temperatures. Conditionally dormant seeds germinated to high percentages at 5 and 15/6°C, and non-dormant seeds germinated to high percentages at 5, 15/6, 20/10, 25/15 and 30/15°C. Seeds that became conditionally dormant at 5°C afterripened completely (i.e. became non-dormant) after transfer to 30/15°C. Buried seeds that became non-dormant in a non-temperature-controlled glasshouse during summer were still non-dormant after 12 weeks of storage at 30/15°C, while those stored at 5°C for 12 weeks had entered conditional dormancy. Thus, low temperatures cause reversal of the afterripening that takes place at high temperatures, but not that which takes place both at low and at high temperatures. Low winter temperatures cause dormant autumn-produced seeds and non-dormant seeds in the soil seed pool to become conditionally dormant. The ecological consequences of these responses to temperature are discussed in relation to the timing of seed germination in nature.     

Seasonal changes in germination responses of buried seeds of the weedy summer annual grass Setaria glauca

Seeds of Setaria glauca (L.) Beauv. buried in soil and exposed to natural temperature cycles exhibited seasonal changes in temperature, but generally not light; dark requirements for germination. Seeds were dormant at maturity in late September and October (autumn), and during burial from October to January they entered conditional dormancy, germinating up to ≥60% in light and darkness at daily thermoperiods of 25/15,30/15 and 35/20^C by January. During burial from February to May or June, seeds became non-dormant and germinated up to 68–100% in light and darkness at 15/6,20/10,25/15,30/15 and 35/20^C in May or June. At maximum yearly temperatures in June or July–August, 65–89% of the seeds entered conditional dormancy (germinating at 30/15 and 35/20, but not at 15/6,20/10 and 25/15^C), and the others entered dormancy (not germinating at any thermoperiod). Thus, most buried seeds had an annual conditional dormancy/non-dormancy cycle, but some had an annual dormancy/non-dormancy cycle. Except for seeds buried in 1990 that lost the ability to germinate in darkness at all thermoperiods the first summer of burial, seeds incubated in light and in darkness exhibited the same patterns of seasonal changes in germination responses. Although conditionally dormant and non-dormant seeds germinated to high percentages in darkness in Petri dishes, seedlings were found only in bags of seeds exhumed in April and May 1983, indicating that some factor(s) associated with the burial environment other than darkness prevented germination of buried seeds.     

The role of light and alternating temperatures on germination of Polygonum aviculare seeds exhumed on various dates

The annual dormancy cycle was investigated in buried seeds of Polygonum aviculare L. exposed to natural temperature changes in Lexington, Kentucky, U.S.A. Seeds were exhumed monthly from December 1984 to February 1987 and tested in light (14-h daily photoperiod) and continuous darkness at 12/12-h daily alternating temperature regimes of 15/6, 20/10, 25/15, 30/15 and 35/20°C. During autumn and winter, seeds became non-dormant, and in March 1985 they germinated to 95-100% at all thermoperiods in light and to 7-61% in darkness. Seeds remained non-dormant during spring but became more specific in their germination requirements in early summer. During July and August 1985, seeds germinated to 17-53% in light at 30/15 and 35/20°C but to 0-10% at all other test conditions. By September, about 65% of the seeds were dormant, but the others were able to germinate under the higher alternating temperatures in light. A similar seasonal cycle was recorded in the following year through to the spring of 1987. The results confirm the seasonal pattern of dormancy in this species (Courtney, 1968) but indicate that alternating temperatures combined with light are important in determining germination potential in P. aviculare.     

Seasonal changes in the germination responses of buried Lamium amplexicaule seeds

Spring-produced seeds of Lamium amplexicaule L. were buried in pots of soil in an unheated glasshouse in June 1978, and at 1–2-month intervals, for 27 months, they were exhumed and tested for germination in light and darkness at temperatures simulating those in the habitat from early spring to late autumn. Freshly-matured seeds were dormant, but by autumn 85% or more germinated in light at 15/6, 20/10, 25/15 and 30/15°C but only 7% or less in darkness. During late autumn and winter germination in light decreased at 25/15 and 30/15 °C but not at 15/6 and 20/10 °C, and germination in darkness increased at 15/6 and 20/10 °C. During late winter and early spring germination in light at 15/6 and 20/10 °C decreased, and seeds lost the ability to germinate in darkness. By the second autumn of burial, seeds germinated to near 100% in light at 15/6 to 30/15 °C and to 10–25% in darkness at 15/6 and 20/10 °C. The cycle of germination responses was repeated during the second winter and spring and the third summer of burial. Autumn-produced seeds were dormant when buried in November 1979, but by spring they germinated to 81 and 36% at 15/6 and 20/10 °C, respectively, in light. These seeds afterripened further during summer. The consequence of seasonal changes in germination responses is that (1) seeds can germinate in the habitat in late summer, autumn and spring but not in early- to mid-summer or in late autumn and winter and (2) during both germination seasons, seeds produced during the previous spring(s) and/or autumn(s) can germinate.     

A high temperature requirement for after ripening of imbibed dormant Poa annua L. seeds

Freshly harvested seeds of Poa annua L. collected in south Louisiana were stored in moist soil at seven temperatures between 5°C and 35°C. At monthly intervals, seed lots were removed and germinated at each of the seven temperatures. Seed were dormant for at least 1 month at all test temperatures. Seeds stored for 2 months at 30 and 35°C showed conditional dormancy; there was 100% germination at 10 or 15°C, and poorer germination at 5 or 20°C. Seeds started to lose viability after 2 months at 35°C and were dead after 7 months. In seeds stored at 10–30°C, there were increased percentages and a wider range of germination temperatures as storage time or storage temperatures increased. Seeds stored at 10°C remained dormant for 9 months, but by 12 months of storage the seeds germinated only at 5 or 10°C. Nearly all seeds stored at the same temperatures in air dry soil remained dormant for 6 months, regardless of storage temperature. These results differ from other reports of low temperatures breaking seed dormancy in Poa annua L. and suggest an adaptation to subtropical climates.     

Role of temperature in regulating timing of germination in soil seed reserves of Thlaspi arvense L.

Summary. Most freshly-matured seeds of Thlaspi arvense L. (Brassicaceae) were dormant at maturity in May. Seeds sown on soil germinated in autumn and spring, but mostly in autumn. Buried seeds exhumed at monthly intervals and tested in light and darkness over a range of thermoperiods exhibited annual dormancy/non-dormancy cycles. However, the dormant period was short, usually only in April, but sometimes May, and in some years 1–6% of the seeds remained conditionally dormant. After-ripening occurred during summer, and seeds were non-dormant during autumn. Seeds entered conditional dormancy in winter and dormancy in late winter or early spring. When buried dormant seeds were kept at 25/15, 30/15 or 35/20°C for 12 weeks, they gained the ability to germinate to 95–100% at 15/6, 20/10, 25/15, 30/15 and 35/20°C. After burial for 12 weeks at 15/6 and 20/10°C, seeds germinated to 80–100% at 15/6, 20/10 and 25/15°C. but to only 11–64% at 30/15 and 35/20°C. After 4 weeks at 5°C, initially-dormant seeds germinated to 100% at all thermoperiods except 35/20°C, where only 15% of them germinated. However, after 18 weeks at 5°C, only 0–1% of the seeds germinated at all thermoperiods. Most non-dormant seeds exposed to 1, 5 and 15/6°C for 16 weeks were induced into dormancy; 1–15% entered conditional dormancy and thus germinated only at 15/6, 20/10 and 25/15°C. This study indicates that seeds of winter annual plants of T. arvense are non-dormant in autumn and enter dormancy in winter, while those from summer annuals are dormant in autumn and become non-dormant during winter.     

Field emergence and temperature requirements for germination in Solanum sarrachoides Sendt.

Emergence of Solanum sarrachoides began in late April, reached a peak in May or June and ceased in September. This pattern closely resembled that for S. nigrum L., whereas almost all seedlings of S. dulcamara L. emerged in April. Fresh seeds of S. sarrachoides were dormant but developed a capacity for germination at 25 and 30°C and at alternating (16 h low/8 h high) temperatures of 4/25, 10/25, 10/30 and 20/30°C when stored dry. kept moist at 4°C or buried in the field. Buried seeds also became capable of germinating at 10. 15 and 20°C and the temperature range for germination was widest during April-June. Induced dormancy developed during August and the range narrowed. The consistent seasonal emergence pattern appears to be associated with cyclic changes in the dormancy status of buried seeds.     

Dormancy studies in three populations of Poa annua L. seeds

Seeds of Poa annua from original collections in Louisiana, Maryland and Wisconsin were grown together in Louisiana over a 3-year period. The freshly harvested seeds and samples stored in moist soil at 30°C were tested for germination at a range of temperatures to compare dormancy and germination characteristics. Seeds of the Louisiana population were dormant over the germination temperature range of 5–25°C, and imbibed storage for 2 weeks did not break dormancy. Freshly harvested seeds of the Maryland population germinated well (78%) at 10°C. With 1 week of imbibed storage at 30°C, germination was good over the range from 5 to 15°C and near 50% at 20°C. Storage for 2 weeks had little further effect. Freshly harvested seeds of two Wisconsin populations germinated above 50% throughout the range of temperatures, and imbibed storage for 2 weeks at 30°C had no effect on germination. The variations in the dormancy of freshly harvested seeds and the varying responses of dormancy breaking from storing imbibed seeds at 30°C suggests that these populations have adapted to avoid high summer temperatures in Louisiana and Maryland but to grow as a summer annual in Wisconsin.     

Germination responses of buried seeds of Capsella bursa-pastoris exposed to seasonal temperature changes

Buried seeds of Capsella bursa-pastoris exhibit an annual conditional dormancy/non-dormancy cycle. Seeds after-ripen during summer and remain non-dormant during autumn and winter. Seeds enter conditional dormancy in early spring, first showing marked decreases in ability to germinate at high (35/20°C) and then at lower (30/15, 25/15°C) temperatures. Seeds do not lose the ability to germinate to high percentages at March (15/6°C) and April (20/10°C) temperatures in March and April. Thus, C. bursa-pastoris is a facultative winter annual, germinating in both autumn and spring if seeds are exposed to light. However, because some seeds retain the ability to germinate at 30/15 and 25/15°C, they could do so throughout the growing season in regions with cool, moist summers. Conditional dormancy developed in all seeds given 12 weeks at 5°C and subsequently kept for 4 weeks each at March (15/6°C), April (20/10°C) and May (25/15°C) temperatures. Thus, seeds of C. bursa-pastoris enter conditional dormancy as temperatures increase in spring.     

Ecological aspects of seed dormancy-break and germination in Heteranthera limosa (Pontederiaceae), a summer annual weed of rice fields

Summary Heteranthera limosa seeds were buried in flooded and in non-flooded soil and exposed to natural seasonal temperature changes in Lexington, Kentucky, USA. Seeds exhumed after various periods of burial ranging from 2 to 36 months were tested for germination under both flooded and non-flooded conditions. Seeds were dormant at maturity in September and became non-dormant during winter. Seeds buried in non-flooded soil during winter germinated to higher percentages and over a wider range of temperatures when tested under flooded conditions (in light) during spring and summer, than did those buried in flooded soil during winter. Thus, the water regime associated with rice culture (non-flooded in winter and flooded in summer) is optimal for dormancy-break and germination of H. limosa seeds. A portion of the buried seeds exhibited an annual dormancy/non-dormancy cycle, whereas others had a conditional dormancy/non-dormancy cycle. Regardless of the type of cycle, seeds buried in non-flooded soil retained the ability to germinate in light at high temperatures under flooded conditions throughout the summer. Thus, seeds potentially can germinate at any time during the growing season, whenever rice fields are flooded. Flooding fields during winter and/or sowing rice relatively early in the growing season may help in establishing rice before seeds of H. limosa germinate.     

Germination ecology of seeds of the annual weeds Capsella bursa-pastoris and Descurainia sophia originating from high northern latitudes

Low temperatures may inhibit dormancy break in seeds of winter annuals, therefore it was hypothesized that seeds of Capsella bursa‐pastoris and Descurainia sophia that mature at high latitudes in late summer–early autumn would not germinate until they had been exposed to high summer temperatures. Consequently, germination would be delayed until the second autumn. Most freshly matured seeds of both species collected in August and September in southern Sweden were dormant. After 3 weeks of burial at simulated August (20/10°C) and September (15/6°C) temperatures, 28 and 27%, respectively, of the C. bursa‐pastoris and 56 and 59%, respectively, of the D. sophia seeds germinated in light at 15/6°C. In contrast, in germination phenology studies conducted in Sweden, only a few seeds of either species germinated during the first autumn following dispersal. However, there was a peak of germination of both species the following spring, demonstrating that dormancy was lost during exposure to the low habitat temperatures between late summer and early autumn and spring. Nearly 100% of the seeds of both species subjected to simulated annual seasonal temperature changes were viable after 30.5 months of burial. In the burial study, exhumed seeds of C. bursa‐pastoris were capable of germinating to 98–100% in light at the simulated spring–autumn temperature regime (15/6°C) in both spring and autumn, while those of D. sophia did so only in autumn. In early spring, however, seeds of D. sophia germinated to 17–50% at 15/6°C. Thus, most seeds of these two annual weeds that mature in late summer do not germinate in the first autumn, but they may do so the following spring or in some subsequent autumn or spring.     

Temperature requirements for germination of buried seeds of Aethusa cynapium L.

Freshly-collected mature mericarps of Aethusu cynapium were dormant, but some germinated at alternating (16 h low/8 high) temperatures when the seed coverings were removed. Burial during winter increased percentage germination and the temperature range over which it took place. In late spring the range narrowed, first at low and then at higher temperatures, widening again in autumn. Moist storage at both low (4°C) and high (30°C) temperatures overcame dormancy, but exposure to 30°C inhibited subsequent germination at low temperatures. Germination of intact mericarps was consistently lower than that of de-coated seeds. The cyclic change in dormancy status of the seeds appears to interact with the restricting effects of the seed coverings and perhaps other factors in determining the consistent pattern of spring emergence in A. cynapium.     

Non-deep simple morphophysiological dormancy in seeds of the weedy facultative winter annual Papaver rhoeas

Embryos in freshly matured seeds of the facultative winter annual Papaver rhoeas are underdeveloped and physiologically dormant; thus, seeds have morphophysiological dormancy (MPD). Seeds lost physiological dormancy during 12 weeks of burial in moist soil at 12 h/12 h daily alternating temperature regimes of 15/5°C, 20/10 °C and 25/15 °C but not at 1 °C. Physiological dormancy was not broken in seeds stored dry at room temperature for 12 weeks. After physiological dormancy was broken, seeds required light for embryo growth (i.e. for loss of morphological dormancy) and consequently for germination. After a 12-week period of burial in soil at 25/15 °C, seeds that matured in 1997 germinated to 100% in light at 25/15 °C, demonstrating that cold stragification temperatures (≈ 0.5–10 °C) are not required for embryo growth. Thus, seeds have non-deep simple MPD. During exposure to low winter temperatures (5/1 °C, 1 °C), 52% of the seeds with physiologically non-dormant embryos entered conditional dormancy and thus lost the ability to germinate at 25/15 °C but not at 15/5 °C or 20/10 °C. The peak of germination for seeds sown in southern Sweden was in autumn, but some also germinated in spring. A higher percentage of seeds that matured in a relatively warm, dry year (1997) came out of MPD and germinated than did those that matured in a relatively cool, wet year (1998) at the same site.     

Seed dormancy and periodicity of seedling emergence in Veronica hederifolia L.

Emergence of Veronica hederifolia seedlings began in mid-October and continued into spring; few appeared from June to September. Ripe seeds shed in June were dormant but wben buried in soil outdoors developed a capacity for germination initially at low temperatures (constant4 C; daily alternations of 4-10° and 4-1 5 C) and later at somewhat higher temperatures, with peak germination in September-November. During winter, spring and early summer thc germination capacity declined, to increase again in late summer and early autumn. Cyclic physiological changes thus occur in seeds of V,hederifolia present in the soil, with which lhe consistent seasonal periodicity of seedling emergence is associated. In dry storage ihe capacity for germination progressively increased, but alter 12 months there was a sharp decline in germination at 4° C. Few seeds germinated at 20° C, but moistening with GA 4/7; brought about complete germination at this temperature.     

Evolution de l'aptitude à germer des graines d'Amaranthus retroflexus L. récoltées dans différentes conditions, au cours de leur conservation

Development during storage of germinability of seeds of Amaranthus retroflexus L. harvested under different conditions The effect was studied of dry storage at 20 ± 1°C for 6 months or in soil 15 cm below the surface during one winter, on the germination behaviour of seeds of Amaranthus retroflexus L. harvested at the level of the main inflorescence on the parent plants grown under natural conditions or in different conditions of controlled photoperiod or temperature. At harvest, the seeds from plants which had developed late (July) in natural conditions were less dormant than those from plants appearing earlier (April); in controlled conditions, plants grown at 20°C in long days (16 h) produced seeds more dormant than those harvested from plants grown either at 20°C in short days (8 h) or at 25°C in long days (16 h). After dry storage or in the soil, this variation in germinability decreased but was never totally suppressed; the seeds retained the characteristics acquired during their formation and maturation. At harvest, for a defined growing condition, the dormancy of the seeds produced depends on the physiological state of the parent plants; after storage, the seeds which were the most dormant at harvest germinated more than the less dormant seeds. Finally, burying has a more favourable effect on breaking dormancy of the seeds than has dry storage.     

Seasonal changes in the germination of buried seeds of Monochoria vaginalis

This study investigates the seasonal variation of germination ability of buried seeds of Monochoria vaginalis (Burm.f.) Presl var. plantaginea Solms. The field-collected seeds were buried in a flooded or an upland field and then exhumed monthly. The exhumed seeds were germinated under four temperature regimes. The seeds exhumed from the flooded soil were dormant at the beginning of burial and proceeded into a conditional dormancy/non-dormancy/conditional dormancy cycle throughout the remaining period of the experiment. The seeds exhumed monthly from the non-flooded soil exhibited an annual dormant cycle, which is dormancy/conditional dormancy/non-dormancy/conditional dormancy/dormancy. At day and night temperatures of 25/20 °C, the exhumed seeds from both the flooded and the upland soil resembled each other in terms of seasonal variation of the germination percentage. In September and October, more seeds exhumed from upland soil failed to germinate under higher temperature than from flooded soil. Strictly avoiding exposure to light during seed exhuming and seed testing prevented the seeds from germinating. A short exposure of the exhumed seeds to light during preparation promoted dark germination when the seeds were at the non-dormant stage. The potential implications of our results for weed management strategies in rice production are discussed.     

Annual cycles of germinability and differences between primary and secondary dormancy in buried seeds of Echinochloa crus-galli

The seasonal changes in percentage of dormant seeds of Echinochloa crus-galli in the field were recorded for 4 years. The lots of seeds were wrapped in nylon fabric, buried 20 cm under the grass sward and exhumed at monthly intervals. The proportion of seeds germinating under light conditions at a constant temperature of 25 °C fluctuated between 0% and 96%, with maxima in May–July and minima in September–November. Small between-year differences in the course of summer dormancy induction and its winter termination were probably caused by variation of weather conditions.
Attributes of dormancy innate to seeds after maturation (primary dormancy) and dormancy induced in buried seeds during the summer (secondary dormancy) were compared by investigating the rate of dormancy termination during storage of (a) dry seeds at 25 °C, (b) imbibed seeds at 5°C and (c) in seeds buried under field conditions during October–June. Percentage of germination increased faster in secondary than primary dormant seeds at both constant 25 °C and 5 °C. The seeds with primary and secondary dormancy also differed in the response to `germination pre-treatment', a 10-day exposure of imbibed seeds at 25 °C that causes germination of the non-dormant fraction of seed materials. After this treatment the time to resuming germination in primary dormant seeds was substantially increased, whereas the secondary dormant seeds were much less affected. Annual variation in the proportion of germinable seeds explains the low efficiency of autumn soil cultivation for decreasing reserves of E. crus-galli seeds in the soil.     

Variation in the annual dormancy cycle in buried seeds of the weedy winter annual Viola arvensis

Seeds of Viola arvensis collected in different years and in different months within those years were buried in soil under natural seasonal temperature cycles, and changes in their germination requirements monitored. Seeds were dormant at maturity in May or June, but nondormant by autumn. During winter, some seeds entered dormancy, while others entered conditional dormancy, i.e. retained the ability to germinate at 15/6 and 20/10^oC but not at other thermoperiods. Dormant and conditionally dormant seeds became nondormant the following summer. Seeds collected in 1981 exhibited an annual dormancy:nondormancy cycle, while those collected in 1982 exhibited an annual conditional dormancy:nondormancy cycle. The type of dormancy cycle found in these seed lots during their first year of burial persisted in subsequent years. Thirty–five and 36% of seeds collected in May 1983 and 1986, respectively, were conditionally dormant the following May, while only 5 and 9% of those collected in the same field in June 1983 and 1986, respectively, were conditionally dormant. Dormant seeds collected in 1981,1982 and 1984 and buried at 5^oC during summer germinated to 0, 33 and 0% respectively, at 15/6^oC in autumn. After the 1982 seeds became nondormant during summer, only 25% entered conditional dormancy when buried at 5^oC, but after the 1981 and 1984 seeds became nondormant, 100% entered conditional dormancy at 5^oC. Thus, the persistent seed bank of V. arvensis at a population site may consist of seeds with an annual dormancy:mondormancy cycle and others with an annual conditional dormancy:nondormancy cycle. This is the first report of the two types of annual seed dormancy cycles in the same species.     

Germination characteristics of Amaranthus quitensis as affected by seed production date and duration of burial

Thermal requirements for the germination of Amaranthus quitensis, a common annual weed in Argentina, were studied. In addition, temporal changes in dormancy from seeds produced at different times during the growing season were examined. For this second objective, thermal and light requirements for germination were tested in seeds buried at different depths, with or without crop residues. Base and optimum temperatures for germination rates were 12.8°C and 37°C respectively. At dispersal time, maximum percentage germination was 60–70% and this was generally recorded at 35°C/25°C in a 14-h photoperiod. Seed germination tended to increase in later seed collection dates. Seeds of A. quitensis showed seasonal changes in germinability in the soil. In winter, germination of retrieved seeds increased to over 90% until summer, after which there was a decrease until the following winter when germination was close to 40%. There were no differences in germinability between burial depths and crop residue levels. Germination requirements for alternating temperatures and light tended to disappear after burial. Initial viability was 99% and declined slightly during burial. Soil temperature seems to play a crucial role not only by regulating seasonal changes in dormancy, but also by defining the percentage and the germination rate in non-dormant seeds.     

Inter‐ and intrapopulation variation in dormancy of Oryza sativa (weedy red rice) and allelic variation in dormancy‐linked loci

Oryza sativa (weedy red rice), the same species as cultivated rice, is a serious problem in rice production worldwide. Seed dormancy contributes to its persistence. We determined the effect of germination temperature and after‐ripening period on germination capacity (GC) of red rice seeds from Arkansas rice fields in three production zones. We also determined the gene diversity (GD) of dormancy‐linked loci among selected populations. The germination behaviour was evaluated at three temperatures (1°C, 15°C and 35°C) and four after‐ripening periods (0, 30, 60 and 90 days) in two independent experiments. Germination response to temperature and after‐ripening time differed among and within populations in each production zone. Overall, populations from the Delta and Grand Prairie were more dormant than those from White River. Regardless of ecotype or production zone, incubation at 35°C (mean GC = 84–100%) favoured the germination of seeds after‐ripened for 60 days. Germination of these seeds was most variable at suboptimal temperature (15°C), with mean GC ranging from 44 to 97%; at 1°C, none of the seeds germinated. Primary dormancy was released in the majority of populations after 90 days of after‐ripening. Blackhull populations generally had lower mean GC than strawhull populations, regardless of temperature, and required longer after‐ripening time to release dormancy. They also showed a higher inter‐ and intrapopulation variation in germination and after‐ripening than strawhulls and had the highest gene diversity (GD = 0.55–0.58) among test populations. Non‐dormant strawhulls were most distant (D = 0.63) from dormant blackhulls. Ecotype influenced genotypic clustering more than the dormancy trait.